Would be the History of Middle-Earth series, volumes I-IV and X and XI. These are not commentary on the text of the Silmarillion as published, but they contain a great deal of "unused" material that could have been part of the Silmarillion, were early drafts of stories that are included and other similar sorts of things. They're not for everyone since they can be repetitious (multiple versions of essentially the same tale) and have a great deal of commentary by Christopher Tolkien on his father's actual method of writing, timelines and other such things. I'd strongly suggest checking them out from the library or borrowing them from a friend before investing in purchasing them yourself. They are intended for those who have read the Sil and crave more rather than as an aid to an initial reading of the Sil. "I like the kind of literary criticism that tries very hard to understand what the author is saying. I despise the kind that cares only about how the reader responds to it. The first requires a great deal of hard scholarship, ultimately as much as had the writer. The second can be practiced by anyone with a navel into which to gaze." ~Reverend Brian Smith